This research will be done primarily in Constanta, Romania as an extension of NIH grant #P30-AI36211 Core F. A cohort study design will be used. The long-term objective of this research study is to determine prognostic factors of HIV disease progression and survival of 343 members of a previously defined cohort. The long-term goal of this research is to enhance the health of infants, children and youth with HIV/AIDS in Romania. Study subjects are 1 to 18 years of age who were diagnosed with HIV infection from 1995 through March 2000 and live in Constanta County in Romania. The specific aims are: Specific Aim 1: to evaluate the relationships of laboratory markers of HIV disease in a cohort of 343 infants, children and youth who were diagnosed with HIV infection from 1995 through March 2000 to a) prospectively describe serum ICD p24 antigen (ICD p24ag) of the pediatric cohort, b) determine the relationship of ICD p24ag with HIV disease progression and mortality, and c) compare the prognostic value of ICD p24ag with or without serum HIV p24 antibody (p24ab) concentration to that of serum HIV RNA concentration with CD4+ lymphocyte percent; and Specific Aim 2: to investigate the predictive patterns of the same laboratory markers among the treatment regimens available in the developing country - Romania. We propose to test the hypothesis that the use of p24 antigen assays, alone or in combination with p24Ab, for assessment of risk of disease progression and mortality in HIV-infected infants and children are both useful and cost effective. This hypothesis will be tested using data (clinical, laboratory, demographic, socioeconomic) collected at every 6-month visit on each member of the cohort. In the past, it was not possible to measure these proposed laboratory markers due to limited resources. With this grant and with the collaboration of the Stefan S. Nicolau Institute of Virology in Bucharest (an International Core laboratory of the Center For AIDS Research at Baylor College of Medicine, Houston, TX), the proposed laboratory markers can be measured. The use of p24 antigen with p24 antibody to identify children at high risk of HIV disease progression or mortality could be a useful alternative to plasma HIV RNA and CD4+ assays in resource poor developing countries. The collaboration, both out of country, Romania and the US, and within country, Constanta and Bucharest, will greatly enhance the research capability of the foreign collaborator. This grant will serve as a model upon which future research studies within Romania can be designed and conducted.